Find out how much weight you should gain during pregnancy based on your pre-pregnancy BMI and current gestational week, per IOM 2009 guidelines.
| Trimester | Period | Approx. Gain |
|---|---|---|
| 1st Trimester | Weeks 1–13 | 1–4.5 lb (0.5–2 kg) |
| 2nd Trimester | Weeks 14–26 | ~1 lb/week |
| 3rd Trimester | Weeks 27–40 | ~1 lb/week |
The Institute of Medicine (IOM) 2009 guidelines recommend different amounts of total weight gain depending on your pre-pregnancy Body Mass Index (BMI).
| Pre-pregnancy BMI | Single Baby (lb) | Twins (lb) |
|---|---|---|
| Underweight (< 18.5) | 28–40 lb | 50–62 lb |
| Normal (18.5–24.9) | 25–35 lb | 37–54 lb |
| Overweight (25–29.9) | 15–25 lb | 31–50 lb |
| Obese (≥ 30) | 11–20 lb | 25–42 lb |
Note: These are general recommendations. Always consult your healthcare provider for personalized guidance during pregnancy.
How much weight should I gain during pregnancy? In the Institute of Medicine's 2009 guidelines the answer depends mostly on one number, your pre-pregnancy BMI: 25 to 35 lb for a healthy-weight BMI with one baby, but 11 to 20 lb if BMI was 30 or higher. This page works out that BMI from your height and pre-pregnancy weight, shows the recommended range in pounds and kilograms, and estimates where you might be at a given week.
Take a woman 165 cm tall who weighed 65 kg before pregnancy and is now at week 20 with one baby. Her BMI is 23.9, which sits in the 18.5 to 24.9 band, so the recommended total gain is 25 to 35 lb (11.3 to 15.9 kg), and the page estimates about 12.2 lb (5.5 kg) at week 20.
How each step is produced. BMI = weight in kg ÷ height in metres squared, so 65 ÷ 1.65² = 23.9, rounded to one decimal. In US mode the page converts pounds with × 0.453592 and inches with × 2.54, so 143 lb at 5 ft 5 in comes out at 23.8. Conversion rounding can move BMI by 0.1.
The category and range come straight from the IOM 2009 table, which ACOG and the CDC both refer to. Single baby: under 18.5, 28 to 40 lb; 18.5 to 24.9, 25 to 35; 25 to 29.9, 15 to 25; 30 or above, 11 to 20. Twins: 37 to 54 lb (normal weight), 31 to 50 (overweight) and 25 to 42 (obese), which the IOM labeled provisional. The IOM gave no twin range for an underweight BMI; the 50 to 62 lb shown for that case on this page is not an IOM figure.
The week-by-week number is not from the IOM. It is a straight-line model: take the midpoint of the range (30 lb here), assign 20 percent to the first 13 weeks, and spread the other 80 percent across weeks 14 to 40. At week 13 that is 30 × 0.2 = 6.0 lb. After that it adds 30 × 0.8 ÷ 27 = 0.889 lb a week, so week 20 is 6.0 + 7 × 0.889 = 12.2 lb. Week 28 gives 19.3 lb and week 40 gives 30.0 lb.
Two cautions about that model. The 6 lb it assigns to the first trimester is above the 1 to 4.5 lb in the page's own trimester table, and because the week field accepts up to 42, the line keeps climbing past week 40 to 31.8 lb at week 42. Treat it as a rough pacing guide, never a target.
Pre-pregnancy weight. The IOM categories are based on BMI before conception, and entering a later, heavier weight can push you into the wrong band.
No. It is population guidance, and your clinician may suggest something different based on your health, history or how the pregnancy is going. Please do not diet or restrict food to chase a number without talking to your provider.
The cut-offs are sharp. A BMI of 24.9 and one of 25.0 sit in different bands, which moves a single-baby range from 25 to 35 lb to 15 to 25 lb. If you are within a few tenths of a boundary, ask your provider which category they use.
The IOM did not issue a range for underweight twin pregnancies. The figure this page shows for that case is not from the IOM, so your clinician's advice should replace it.
Do not try to lose weight on your own. Tell your doctor or midwife, who can look at your overall picture. This page is for education and is not medical advice.